Air-fuel ratio feedback control using a so-called air-fuel ratio sensor, the air-fuel ratio sensor being configured such that an output current value output thereby when a predetermined voltage is applied thereto varies linearly relative to an air-fuel ratio of exhaust gas, is known as air-fuel ratio control performed in an internal combustion engine. When the air-fuel ratio of the exhaust gas is richer than the stoichiometric air-fuel ratio, oxygen in an atmosphere duct of the air-fuel ratio sensor is ionized by an atmosphere side electrode, and when resulting oxygen ions move to an exhaust side electrode through a solid electrolyte layer, a current flows through the air-fuel ratio sensor. Hence, when the air-fuel ratio of the exhaust gas continues to be richer than the stoichiometric air-fuel ratio, leading to a deficiency in the amount of oxygen in the atmosphere duct, a detection value obtained by the air-fuel ratio sensor deviates to a lean side of the actual air-fuel ratio. To prevent this deviation, JP2008-14178A describes temporarily terminating the air-fuel ratio feedback control and switching to open loop control when the detection precision of the air-fuel ratio sensor deteriorates in the manner described above.
However, when the air-fuel ratio feedback control is terminated in the manner described in the above document, it is no longer possible to absorb variation in a fuel injection amount caused by individual differences in components such as a fuel injection valve. Therefore, the control precision is poorer during the open loop control than during the air-fuel ratio feedback control, and as a result, various performance indices, such as engine output, fuel efficient, and exhaust emissions, deteriorate.